1. Field of the Invention
The invention relates to a system for measuring flow rate with at least one first flow meter and one second flow meter.
2. Description of Related Art
Aside from the fact that high measurement accuracy in any flow rate measurement is always desirable from a technical standpoint, for high-value liquid or gaseous products, such as, for example, petroleum and natural gas, there is considerable interest in the fact that, from the standpoint of the supplier, the amount and only the amount which is to be delivered is delivered, and from the standpoint of the consumer, that the required amount and at least the required amount is obtained. Measurement tolerances are always a burden to one of the parties involved, usually to the burden of the supplier.
Known measurement methods for determining the flow rate use, for example, the transit time of ultrasonic waves, the time being transmitted either directly between the two ultrasonic transducers or undergoing a reflection on the wall of the measurement tube (see, for example, German Patents DE 196 48 784 C2 and DE 103 12 034 B3). For this application, especially flow meters with a measurement accuracy as high as possible are preferred. Ultrasonic measurement methods can be used advantageous especially for large pipe widths since, with the ultrasonic waves in a suitable arrangement of the ultrasonic transmitters and receivers, the flow-traversed pipe cross section can be essentially completely monitored; this applies mainly to those measurement devices which implement a plurality of measurement paths which traverse the flow cross section in different regions.
Based on the changing measurement conditions or ageing phenomena of the measurement devices, however, measurement uncertainties or even measurement errors can arise. In order to identify these phenomena, the prior art discloses placing different flow meters in succession, the measurement devices differing from one another in measurement principle or by the manufacturer. This is associated with the expectation that the measurement devices, for example, show different ageing behavior and react differently to changes of measurement conditions (for example, dirt or deposits). To the extent this is given and if deviations of measurement data are observed and recognized, this diversity of the measurement of the flow rate is used for monitoring of the system.
The disadvantage in these systems is that the diversity must be combined again; this can be associated with difficulties. If different measurement principles are used, the values cannot be compared directly to one another or different error tolerances must be observed. Moreover, if measurement devices of different manufacturers are used, optionally, additional information about the respective particulars must be present. Therefore, it would be advantageous to have a simplified system so that computational effort can be concentrated on the actual evaluation and not for comparing of the data from multiple devices.
The initially described applications of flow rate measurement are often the so-called standardization-mandated trade in which special demands are imposed on the accuracy of the measurement devices and mainly on the demonstration of this accuracy. For the demonstration of the accuracy, it is generally necessary to remove the system—therefore, the measurement devices with their respective measurement tubes and optionally also connecting tubes between them—from the surrounding pipe system at regular intervals and to subject them to calibration, here conformity with certain accuracy requirements being confirmed by institutes authorized to do this, such as, for example, the Federal Physical-Chemical Institute in Germany. This process is very complex since these systems can be heavy and since provisions must be made for the required bypasses. Therefore, it would be advantageous to use systems which have calibration intervals as long as possible, ideally, after a first calibration, have an “infinite” calibration term.